DPFs have been known as an effective technology for removing PM in exhaust gas from a diesel engine.
The DPF is a PM collecting device using a filter and is arranged in an exhaust path. The DPF is configured to collect PM such as soot exhausted from the engine by a filter and then remove PM from the exhaust gas. A part of the PM collected by the DPF is combusted by exhaust gas of high temperature exhausted from the engine (natural regeneration), and the rest of the collected PM accumulates in the DPF. When the accumulation of PM progresses excessively, PM collection performance declines and the engine output declines. Therefore, in DPFs, it is necessary to perform an active regeneration at an appropriate timing to actively burn the PM accumulated in the filter and regenerate the filter.
To determine an appropriate timing for performing the active regeneration, it is necessary to accurately estimate a PM accumulation amount of the filter. If the PM accumulation amount is underestimated, the active regeneration timing is delayed. This causes decline of the PM collection performance, decline of the engine output, and so on, and possibly causes damage to the DPF due to excessive temperature rise during the active regeneration. In contrast, if the PM accumulation amount is overestimated, the active regeneration is performed frequently and issues such as deterioration of fuel economy and oil dilution.
The formula for estimating the accumulation amount of PM accumulated in the filter of the DPF is typically represented by the following formula (1).PM accumulation amount=PM emission amount−PM regeneration amount  (1)
The PM emission amount here is an amount of PM contained in the exhaust gas exhausted from the engine. Further, the PM regeneration amount particularly is a passive regeneration amount, which is an amount of PM burned by the exhaust gas of high temperature exhausted from the engine during normal operation and not during the active regeneration.
The above PM emission amount is computed using a map having an engine rotation speed and a fuel injection amount as input data. On the other hand, the PM regeneration amount is computed based on measured values from a variety of sensors such as a temperature sensor, a pressure sensor, an airflow meter, in addition to the engine rotation speed and the fuel injection amount. Thus, if a sensor such as the airflow meter fails, it is difficult to estimate the PM regeneration amount.
Patent Reference 1 discloses a technology for avoiding excessive accumulation of PM in a filter when sensors necessary for estimating the PM regeneration amount, such as the airflow meter fails. According to Patent Reference 1, in such case, the PM accumulation amount is computed as PM accumulation amount≈PM emission amount without computing the PM regeneration amount according to the above formula (1) so as to prevent underestimation of the PM accumulation amount.